A heavy fog blanketed parts of northern China on Monday, delaying flights and causing hundreds of cancellations, while smog hung in a dark haze over Beijing.

As of 2 p.m. (0600 GMT), 126 flights had been delayed by an hour or longer and 207 were cancelled at Beijing, the world's second-busiest airport, Xinhua news agency said.

The Beijing sky was so dark that many drivers kept their headlights on throughout the day, giving the city an eerie, netherworld feeling.

Beijing night covered by smog 4th Dec. 2011.

West 5th ring road of Beijing, morning 3rd Dec. 2011.

4th Dec. 2011, Qianmen (front gate) in smog.

"Such super foggy weather looks like the end of the world," commented one microblogger using the name David Jiaoxiaomao.

China's national weather forecaster said the fog was likely to persist across parts of China to Wednesday, causing more transport disruptions. By then, a cold front would begin dispersing the fog, said the forecaster, according to Xinhua.

Highways across the northern provinces of Shandong and Hebei were also closed.

In Beijing, the fog has been made worse by pollution. Readings by the U.S. Embassy, which measures inhalable particles of 2.5 microns, have described the pollution for days as "hazardous".

On Sunday night, the U.S. Embassy's index topped its ceiling of 500, and it was 356 on Monday afternoon, a reading that was still considered "dangerous".

The Beijing environmental bureau, however, said the air was affected by only "light pollution" during the day.

Chinese environmental officials have come under criticism in media reports and on microblogs for reporting only measurements of much larger particles in the air.

Du Shaozhong, a deputy head of the bureau, warned on his microblog that residents should take precautions if pollution rises to "medium" levels, as it has during the night, when the government's central heating furnaces pour smoke into the air.